A typical cellular wireless network includes a number of base stations each radiating to define a respective coverage area in which user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices, can operate. In particular, each coverage area may operate on one or more carriers each defining a respective frequency bandwidth of coverage. In turn, each base station may be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE within coverage of the network may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other UEs served by the base station.
Further, a cellular wireless network may operate in accordance with a particular air interface protocol (radio access technology), with communications from the base stations to UEs defining a downlink or forward link and communications from the UEs to the base stations defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, Orthogonal Frequency Division Multiple Access (OFDMA (e.g., Long Term Evolution (LTE) and Wireless Interoperability for Microwave Access (WiMAX)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), and Global System for Mobile Communications (GSM), among others. Each protocol may define its own procedures for registration of UEs, initiation of communications, handover between coverage areas, and other functions related to air interface communication.
In accordance with a recent version of the LTE standard of the Universal Mobile Telecommunications System (UMTS), for instance, each coverage area of a base station may operate on one or more carriers spanning 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz, with each carrier being divided primarily into subcarriers spaced apart from each other by 15 kHz. Further, the air interface is divided over time into a continuum of 10-millisecond frames, with each frame being further divided into ten 1-millisecond subframes or transmission time intervals (TTIs) that are in turn each divided into two 0.5-millisecond segments. In each 0.5 millisecond segment or in each 1 millisecond TTI, the air interface is then considered to define a number of 12-subcarrier wide “resource blocks” spanning the frequency bandwidth (i.e., as many as would fit in the given frequency bandwidth). In addition, each resource block is divided over time into symbol segments of 67 μs each, with each symbol segment spanning the 12-subcarriers of the resource block and thus supporting transmission of symbols in “resource elements.”
The LTE air interface then defines various channels made up of certain ones of these resource blocks and resource elements. For instance, on the downlink, certain resource elements across the bandwidth are reserved to define a physical downlink control channel (PDCCH) for carrying control signaling from the base station to UEs, and other resource elements are reserved to define a physical downlink shared channel (PDSCH) for carrying bearer data transmissions from the base station to UEs. Likewise, on the uplink, certain resource elements across the bandwidth are reserved to define a physical uplink control channel (PUCCH) for carrying control signaling from UEs to the base station, and other resource elements are reserved to define a physical uplink shared channel (PUSCH) for carrying bearer data transmissions from UEs to the base station.
In a system arranged as described above, when a UE enters into coverage of a base station, the UE may engage in attach signaling with the base station, by which the UE would register to be served by the base station on a particular carrier. Through the attach process and/or subsequently, the base station and supporting LTE network infrastructure may establish for the UE one or more bearers, essentially defining logical tunnels for carrying bearer data between the UE and a transport network such as the Internet.
Once attached with the base station, a UE may then operate in a “connected” mode in which the base station may schedule data communication to and from the UE on the UE's established bearer(s). In particular, when a UE has data to transmit to the base station, the UE may transmit a scheduling request to the base station, and the base station may responsively allocate one or more upcoming resource blocks on the PUSCH to carry that bearer traffic and transmit on the PDCCH to the UE a downlink control information (DCI) message that directs the UE to transmit the bearer traffic in the allocated resource blocks, and the UE may then do so. Likewise, when the base station has bearer traffic to transmit to the UE, the base station may allocate PDSCH resource blocks to carry that bearer traffic and may transmit on the PDCCH to the UE a DCI message that directs the UE to receive the bearer traffic in the allocated resource blocks, and the base station may thus transmit the bearer traffic in the allocated resource blocks to the UE. LTE also supports uplink control signaling on the PUCCH using uplink control information (UCI) messages. UCI messages can carry scheduling requests from UEs, requesting the base station to allocate PUSCH resource blocks for uplink bearer data communication.
Moreover, a revision of LTE known as LTE-Advanced now permits a base station to serve a UE with “carrier aggregation,” by which a base station schedules bearer communication with the UE on multiple carriers at a time. With carrier aggregation, multiple carriers from either contiguous frequency bands or non-contiguous frequency bands can be aggregated to increase the bandwidth available to the UE. Currently, the maximum bandwidth for a data transaction between a base station and a UE using a single carrier is 20 MHz. Using carrier aggregation, a base station may increase the maximum bandwidth to up to 100 MHz by aggregating up to five carriers. Each aggregated carrier is referred to as a “component carrier.” Further, when multiple carriers are aggregated, one of the component carriers may be defined as a primary cell (“PCell”) and the remaining component carriers may be defined as secondary cells (“SCells”). A UE served with carrier aggregation may send and receive control signals in the PCell while sending and receiving bearer data in the PCell and the SCells.
In operation, a base station typically includes amplifiers to amplify RF signals, band-pass filters to pass a configured frequency range (i.e., the pass band of the filter), and band-stop filters to block frequencies outside of the configured frequency range. For example, in some LTE deployments, a base station may be configured with filters that pass an entire LTE band (e.g., LTE Band 41) and block frequencies outside of the band. In other LTE deployments, a base station may be configured with filters that pass a portion of an LTE band (e.g., a 50 MHz range of frequencies within LTE Band 41) and block frequencies outside of the desired portion the LTE band (e.g., outside of the desired 50 MHz frequency range).
Signals that traverse the amplifiers and filters of base stations will experience various signal impairments, including group delay, which is a measure of the time delay of the amplitude envelopes of the various sinusoidal components (e.g., subcarriers) of the signal as the signal propagates through a filter and/or amplifier. Group delay is inversely proportional to filter bandwidth and nearly proportional to the order of the filter. In multi-carrier transmission systems, all of the subcarriers of a signal are delayed when the signal propagates through a filter and/or amplifier. However, the delay tends to be frequency-dependent, and thus, the delay will be different for the various subcarriers. For example, subcarriers near the edge of the filter's pass band tend to experience greater group delay than subcarriers in the middle of the filter's pass band.
In LTE networks, group delay variation (GDV) tends to be more problematic than the delay of any individual subcarrier in part because of the way subcarriers are managed and allocated for data transmissions between base stations and UEs. In particular, GDV is the difference between the time delays of the subcarriers of a resource block, or perhaps the difference between the time delays of subcarriers in different resource blocks. These differences in inter-resource block and intra-resource block group delay tend to cause signal distortions that can reduce signal quality and reliability. In practice, such signal distortions may increase when the GDV increases, thereby further reducing signal quality and reliability.